The present invention relates to detecting gaseous species in a mixture by light-emission spectroscopy.
In order to detect gaseous species, recourse has already been made to light-emission spectroscopy, in which use is made of the light radiation emitted by a plasma present in the gas mixture for analysis, the optical spectrum of said radiation emitted by the plasma is measured, and the optical spectrum is analyzed in order to deduce therefrom the presence of gaseous species in the mixture.
The conventional method used for the step of analyzing the optical spectrum consists in viewing the optical spectrum in real time and in comparing it with spectra published in scientific libraries and established for each gaseous species. The method relies on the fact that each gaseous species generates light radiation of spectrum that is characteristic when it reaches a level of excitation causing it to emit light. Scientific libraries thus contain the light-emission spectra for each gaseous species. Each spectrum is constituted by a curve plotting light intensity values as a function of wavelength over the wavelength range constituting light radiation, i.e. in the ultraviolet, in the visible spectrum, and in the infrared. Generally, the light-emission spectrum of a gaseous species is a jagged curve presenting a large number of peaks or “lines”. Each line is characterized by the wavelength and by the intensity of the light radiation and/or wavelength.
In known apparatuses, the light-emission spectrum is generally viewed by means of a computer which scans through the data issued by an optical spectrometer. Software associated with the spectrometer usually makes it possible to act on the integration time of the signal coming from the spectrometer, and thus on the intensity of the spectrum. The software may also act on the number of spectra to be averaged prior to display, thus making it possible to reduce noise. The software then allows the instantaneous light-emission spectrum to be viewed, and allows the variation in the amplitude of certain lines to be tracked, in order to deduce changes in the presence of a gas. The amplitude of a line A1 at wavelength λ1 from a measurement spectrum associated with identifying a species A makes it possible, when the gas is on its own, to monitor variation in the presence of that gas. The software also makes it possible to perform a certain number of mathematical operations such as subtracting spectra.
However, the drawback of those traditional techniques lies in the difficulty of interpreting the resulting spectrum. It is not possible to determine immediately the or each species contained in a mixture. That makes it necessary to consult tables of known spectra and to attempt to recognize the lines of known spectra in the spectrum under analysis. A first line is identified in the spectrum of the literature, then a second, . . . .
Spectrum analysis needs to be performed by an expert, since there are numerous difficulties, and in particular:
in the spectrum under analysis, some of the lines of the gaseous species being sought can be missing, even though the gaseous species is genuinely present; certain lines can be missing as the result, in particular, of the conditions under which the plasma is generated, since the presence of these lines depends on the power of the plasma and on other parameters of the plasma;
in the spectrum under analysis, because the spectrometer is necessarily of limited resolution, it is often difficult to know accurately the characteristic wavelength of a spectrum line, which then makes it difficult to allocate the line to one or another of a plurality of possible gaseous species; this becomes particularly difficult in the presence of lines that are very close together;
certain gaseous species that are easily excited can block out other gases of interest from the display since they do not have enough energy to reach excitation levels at which they emit light; as a result, some or all of the lines of a gaseous species can be missing because of the presence of other gaseous species that are easily excited, and this needs to be taken into account when analyzing the spectrum; and
the software associated with the spectrometer is not capable of reliably interpreting variation in the amplitude of radiation at the characteristic wavelengths of the spectrum; for example, in the presence of a first species A presenting a line A1 of wavelength λ1 as shown in FIG. 3A, the software will track variation in the amplitude at the wavelength λ1; if a second species B is then introduced into the mixture, where said second species has a line B1 at wavelength λ2 close to λ1, as shown in FIG. 3B, then the software will detect an increase in the amplitude of A1 up to A1′ and runs the risk of detecting an increase in the quantity of the species A, as shown in FIG. 3C, whereas, in fact, it is the presence of the species B that explains the increase in the amplitude at wavelength λ1; this shows that it is unsafe to follow variation of a species on the basis of the amplitude of a single line; and a plurality of tracked lines are even more complicated to interpret.
The expert needs to take all of the above difficulties into account in order to extract from the resulting spectrum information that is pertinent about the presence of the various gaseous species in the mixture and how they vary.
Those traditional solutions thus require permanent intervention by an expert in order to analyze the spectra that are obtained, and such analysis is lengthy and tedious, making it inconceivable to perform analysis in real time, e.g. for the purpose of detecting real-time variation in a parameter of the equipment through which the gas mixture under analysis is flowing.
The problem posed by the present invention is to avoid the drawbacks of prior art systems, making it possible automatically and quickly to obtain the composition of a gas mixture being studied by light-emission spectroscopy, without requiring the intervention of an expert.
The invention thus seeks to make it possible to follow the variations in the composition of gas mixtures in real time.
The invention also seeks to automate interpreting the spectra obtained by light-emission spectroscopy apparatus.
For this purpose, the essential idea of the invention is to process the spectrum delivered by a light-emission spectrometer digitally and preferably in real time in order to extract meaningful information therefrom.
To do this, in the raw light-emission spectrum, it is considered that a steep slope is indicative of information that is useful for analyzing gaseous species. Thus, use is made of information in the spectrum that is identified as being in the vicinity of such a steeply sloping zone.
The spectrum is constituted by a plot of values for the intensity of light radiation as a function of the wavelength of the radiation. In practice, the results given by the optical spectrometer are contained in a set of tables, each table representing the spectrum at a sampling instant. Each table is made up of a sequence of intensity values and the simultaneous corresponding sequence of wavelength values.
The table is searched for zones that correspond to rapid variations in the intensity of light radiation as a function of wavelength. For each zone in the table corresponding to such rapid variation in the intensity of light radiation as a function of wavelength, the maximum intensity value in the vicinity of the zone of rapid intensity variation is identified, and the wavelength value corresponding to said intensity maximum is identified.
This defines a pruned optical spectrum table that contains only those maximum intensity values and the corresponding wavelength values that are in the immediate vicinity of zones in which intensity variation is greater than a determined threshold for intensity variation.
By means of this procedure, the raw optical spectrum, initially constituted by several thousands of points covering the spectral range of the spectrometer is simplified and replaced by a pruned optical spectrum containing only about one hundred characteristic points constituted by the maximum intensity values and the corresponding wavelengths for zones in which intensity variation is greater than the determined threshold.
The determined threshold for intensity variation can be adjusted depending on the extent to which it is desired to prune the spectrum.
In usual gas mixtures, a raw optical spectrum is a table made up of a sequence of several thousands of intensity values and of corresponding wavelength values. It will be understood that a comparison relating to such tables having several thousands of values is tedious and time consuming, making real time monitoring inconceivable.